This invention relates to the pulping of waste paper products for the recovery of reusable paper-making fibers therefrom and, more particularly, to methods and apparatus for recovering reusable paper-making fiber from waste paper products containing junk or contaminants.
A problem of increasing magnitude in the pulping of waste paper products has been the steady increase in the amount and nature of the contaminants mixed therewith in commercially obtainable waste paper, the contaminants now commonly averaging of the order of 15% by weight. Of particular importance is the amount of lightweight contaminant junk, primarily in the form of plastics products of many kinds and especially plastics sheet and film and also pieces of plastics foam.
In the past, many of the common contaminants of waste paper could be eliminated from the pulper tub by the use of a junk remover, a typical example being shown in British patent specification No. 1,266,420. Such a junk remover relies on gravity discharge, through a downward chute from the pulper tub, of iron and other junk material of substantially higher specific gravity than paper fibers. But such junk removers have proved to be ineffective for removing lightweight junk for two principal reasons.
One reason is the obvious one that material lighter than water will not readily flow down the chute which connects a pulper tub with its junk remover. The other is that the normal operation of a pulper rotor tends to force sufficient liquid from the tub to the junk remover when the pulping operation commences to maintain a higher static head in the junk remover than in the tub, commonly of the order of 60 or more centimeters (two or more feet). Further, the common practice is to add fresh liquid to the tub by way of the junk remover in order to wash fiber back into the tub from the high specific gravity pieces travelling through the chute from the tub, and this increases the opposite to the flow of light materials from the tub.
The result of these conditions is that when a waste paper pulper--whether or not it is equipped with a junk remover--is operated on a continuous basis, with continuous extraction, through a perforate extraction plate, of a slurry of sufficiently small particle size, and continuous replacement of water and furnish, plastics tend to accumulate in the tub until the amount of extracted fiber drops below an acceptable rate, a condition which the industry calls "constipated". It is then necessary to discontinue pulping and manually empty the accumulated junk from the tub.
The development of this condition has three significant disadvantages. Running of the pulper until the paper fiber can no longer be extracted not only results in loss of production of recovered paper fiber but also produces increased an unnecessary wear on the pulper rotor and its extraction plate. In addition, its results in extraction of a substantial amount of small plastics particles with the paper fiber, as the quantity of plastics in the tub increases to the point where it comes into contact with the rotor, and such small pieces of plastics are difficult to separate from the paper fiber, especially if the holes in the extraction plate are small. At the same time, manual emptying of accumulated plastics is expensive and time consuming, and it also results in the loss of a substantial amount of fiber which remains commingled with the plastics and is therefore eliminated along with the plastics.
British patent specification No. 1,547,284 taught that these disadvantages of past practice can be overcome, and the effectiveness of the junk remover greatly improved, by maintaining the liquid level in the junk remover lower than in the pulper tub and thereby inducing liquid flow from the tub into the junk remover. In accordance with that specification, this is done by connecting the inlet of a pump to the junk remover casing at a level below the minimum operating level in the tub, and withdrawing liquid from the junk remover and recirculating it back to the tub under controlled conditions establishing the desired lower liquid level in the junk remover than in the tub, e.g. lower by about a few centimeters or inches.
The effect of this removal of the normal static head conditions is firstly to induce flow through the chute from the tub into the junk remover. Lightweight trash circulating in the tub will be entrained in that flow and, as soon as it enters the junk remover, it will rise to the top and thus be trapped against return to the tub. The resulting accumulation of lightweight trash at the top of the liquid in the junk remover is lifted out for removal by the perforated conveyor buckets which are standard equipment in a junk remover.
Another solution to the problem, taught in U.S. Pat. No. 4,129,259, lies in the provision of a system operating in combination with a pulper and junk remover wherein the plastics and other lightweight trash picked up by the junk remover conveyor buckets is dumped into a junk box which is continually filled with liquid to a sufficient level to float lightweight trash over a weir leading to a hopper. Detrasher means, in the form of very coarse straining means, such as a grid of tine-like members, is positioned in the path of the overflow from the weir into the hopper, with the tines being so spaced with respect to each other, and at such angle to the horizontal, that they will permit the passage of most of the plastics sheet and similar contaminant material but will shunt large pieces of floating trash, such as particularly chunks of wood or plastics, to a separate receiver.
The material passing through the detrasher grid may be returned directly to the pulper tub for further defibering, or may first be subjected to a deflaking operation, which may be done by a pump capable of such action or by a deflaker in conjunction with a pump capable of handling a fluid flow containing substantial quantities of solids. The output of the deflaking section of the system is then preferably screened to reject large plastics pieces and the like, with the accepts flow from such screening being returned to the pulper tub for further defibering.